[PATCH] compiler_types.h: fix "unused variable" in __compiletime_assert()

[marc . herbert]

From: Marc Herbert <Marc.Herbert@xxxxxxxxxxxxxxx>

This refines commit c03567a8e8d5 ("include/linux/compiler.h: don't
perform compiletime_assert with -O0") and restores #ifdef __OPTIMIZE__
symmetry by evaluating the 'condition' variable in both compile-time
variants of __compiletimeassert().

As __OPTIMIZE__ is always true by default, this commit does not change
anything by default. But it fixes warnings with _non-default_ CFLAGS
like for instance this:

 make  CFLAGS_tcp.o='-Og -U__OPTIMIZE__'

                 from net/ipv4/tcp.c:273:

 include/net/sch_generic.h: In function ‘qdisc_cb_private_validate’:
 include/net/sch_generic.h:511:30:
            error: unused variable ‘qcb’ [-Werror=unused-variable]

  {
     struct qdisc_skb_cb *qcb;

     BUILD_BUG_ON(sizeof(skb->cb) < sizeof(*qcb));
     ...
  }

Signed-off-by: Marc Herbert <Marc.Herbert@xxxxxxxxxxxxxxx>
---
 include/linux/compiler_types.h | 7 ++++++-
 1 file changed, 6 insertions(+), 1 deletion(-)

diff --git a/include/linux/compiler_types.h b/include/linux/compiler_types.h
index 501cffddc2f4..3f12ca49bb9e 100644
--- a/include/linux/compiler_types.h
+++ b/include/linux/compiler_types.h
@@ -525,6 +525,11 @@ struct ftrace_likely_data {
 	 sizeof(t) == sizeof(int) || sizeof(t) == sizeof(long))
 
 #ifdef __OPTIMIZE__
+/* #ifdef __OPTIMIZE__ is only a good approximation; for instance "make
+ * CFLAGS_foo.o=-Og" defines __OPTIMIZE__, does not elide the
+ * conditional code and can break compilation with wrong error
+ * message(s). Combine with -U__OPTIMIZE__ when needed.
+ */
 # define __compiletime_assert(condition, msg, prefix, suffix)		\
 	do {								\
 		/*							\
@@ -538,7 +543,7 @@ struct ftrace_likely_data {
 			prefix ## suffix();				\
 	} while (0)
 #else
-# define __compiletime_assert(condition, msg, prefix, suffix) do { } while (0)
+# define __compiletime_assert(condition, msg, prefix, suffix) ((void)(condition))
 #endif
 
 #define _compiletime_assert(condition, msg, prefix, suffix) \
-- 
2.49.0